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MusictoYourEars:NewTransducersMeetElectrostaticHeadphonesAnaudiotechnologystartupdeliversnewmanufacturabletransducersforhigh-endelectrostaticheadphonesandreduceslow-endroll-off.
ByJenniferHand
Serioushi-fienthusiastsgetexcitedaboutthemusicalexperiencedeliveredbyelectrostaticheadphones.Producinganatural,airysound,theyprovidegreaterclarity,lessdistortion,andextendedbandwidthwhencomparedtoothertypesofheadphoneswherehighresolutionaudiosourcesareinvolved.
Mostelectrostaticspeakersapplyanelectricchargeonathinelasticmembranesituatedbetweentwoconductiveplates.Thechargedmembranemovesindirectresponsetotheelectricalinput,generatingthesoundwavesthatourearsandbraininterpretasmusic,andmovingustojoyandtears.
Despitetheirhighqualityandaccurateaudioreproduction,electrostaticspeakerscanbeprohibitivelyexpensive,sometimesfragile,anduntilrecently,werehandmadebecauseofmechanicalprecisionrequirements.Seeinganeedforaffordable,high-qualityheadphonesthatcouldbemanufacturedmoreeasily,WarwickAudioTechnologiesLimited(WAT)designedtheHigh-PrecisionElectrostaticLaminate(HPEL)transducer,apatentedtechnologybasedonanultrathindiaphragmandasingleconductiveplateinsteadofapair.WithitsoriginsatWarwickUniversityintheUK,WAThasdevelopedalightweightlaminatemembraneonly0.7mmthickthatisperfectlysuitedforelectrostaticheadphones.
ThenewHPELsarelightweightthin-filmstructuresmanufacturedthroughacontinuousrollprocess.“Thetechnologywe’vedevelopedisunique,”explainsMartinRoberts,CEOofWAT.“TheHPELtransducerismadeupofametallizedpolypropylenefilm,apolymerspacerwithhexagonalcells,andaconductivemesh"(Figure1).
Figure1.Lefttoright:WAT’sHPELtransducers;singlelaminate,assembled,andexplodedviewsofa
finishedHPELtransducer.AlllaminatesaremadeintheUK.
IInthetypicalsetup,directcurrent(DC)biasvoltageisappliedtotheelasticmembraneandalternatingcurrent(AC)drivesignaltothesurroundingplates.WAT'sone-sidedspeakerinvolvesboththeDCbiasandtheACdrivesignalappliedtotheelasticmembrane,withasinglewiremesh(plate)positionedoppositethemembraneasagroundplane.
Thefabricationmethodmakesitpossibletoreproducethetransducersatasignificantlylowercostthantraditionalelectrostaticspeakers.Thismeansthatforthefirsttime,electrostaticsmaybe
consideredacommerciallyviablehigh-resaudiooptionacrossawiderangeofdevicetypesandmarketsegments.
SimulatingAcousticPlaybackTodevelopatransducerlikethis,whichcanbeeasilymanufacturedandinexpensivewithout
compromisingsoundquality,theWATteamthoroughlyinvestigatedtheinfluenceofmanydesignelementsbeforesettlingonafinalversion.“Wehaddevelopednumerousprototypesthatclearlyperformed.Thebigissuewasthatwewerenotentirelysurehowvaryingindividualmaterialanddesignparametersaffectedthetransducer’sperformance,”Robertssays.
ThedynamicsoftheHPELaredependentontheextremelycomplexinterplaybetweenmembranetension,ACsignallevel,speakergeometry,elasticanddielectricmaterialproperties,thermoacousticlosses,andtheaddedmasseffectsoftheairnexttotheopensideofthemembrane.Thedesignerswantedtoimprovebassperformancebyreducinglow-endroll-off,minimizingdistortion,andmaximizingthesoundpressurelevelforagivenelectricalinput.Buttheydiscoveredthatsmallchangestoanycomponentgreatlyaffectedtheacousticoutput.
AlthoughWAThadsignificantmechanical,electrical,andacousticexpertise,theyhadnoin-housesimulationcapabilitytohelpthemunderstandthisinterplay.InordertoperformavirtualoptimizationoftheHPELtransducerdesigntheyenlistedthehelpofXiEngineering,aCOMSOLCertifiedConsultantthatspecializesincomputationalmodeling,designrecommendations,andsolvingnoiseandvibrationproblemsinmachineryandothertechnology.
Dr.BrettMarmo,technicaldirectorofXiEngineering,oversawthedevelopmentofthe
COMSOLMultiphysics®softwaremodelstheyusedtoanalyzethebehavioroftheHPEL.COMSOL®software,whichallowedXiEngineeringtomodelnonlineareffectsthatwouldarisewithamendmentstotheHPEL’sasymmetricaldesign.
“Wekepttheearlymodelsimple,focusingonspecificsthatinfluencesoundquality,forexamplekeepingthefirstharmonicaslowaspossibletounderstandtheacoustic-structureinteractionandtheHPEL’sperformanceatlowfrequency,”Marmoexplains,describingtheirpreliminarytests.“Ourmodelshowedhowappliedvoltageaffectssignallevels,whichhelpedusunderstandsounddistortionforaninitialcase.”
Becausethetransducerisone-sided,theelectrostaticforcevarieswiththepositionofthevibratingmembrane,decreasingwiththesquareofthedistancebetweenthemembraneandthemesh.Oncetheyunderstoodtheresultingnonlineardistortionandwereabletopredictitseffects,theWATengineerscouldthencancelanyrelateddistortionselectrically.
PerfectingtheHPELTransducerDesignInamoreextensivesimulationthatinvolvedastructural-MEMS-acousticcoupling,heexamined
theimpactofadjustingparameterslikethesizeofthehexagonalcellsinthewiremesh,thicknessofthewires,membranetension,spacingbetweenmembraneandmesh,andmaterialpropertiesofeachcomponent.MarmoandhiscolleaguesalsostudiedtheeffectsofdifferentDCbiases,whichareoftenresponsiblefordistortionatlowfrequencies,andlookedatconductivityalongtheplatetodiscernwhethervoltageswerehigherinoneareathananother.TheythenusedCOMSOLtostudythethermoacousticlossesandmodelthedisplacementofthemembranefordifferentfrequencies(Figure2).
Figure2.Simulationplotshowingthesoundpressurelevel(thermalcolorsurface)indBandthedisplacementofthemembrane(rainbowcolorsurface)inmmfromafullycoupledacoustics-MEMS
modelsolvedinthefrequencydomain.Left:solutionat5,000Hz.Right:solutionat5,250Hz.
“Wefoundthatthistypeofsimulationwastheonlyaccuratewaytotrulymodelplanarelectrostatictransducers,”Marmocontinues.“Forthiscase,lumpedparametermodelingcancharacterizelimitedaspectsofperformance,suchaslow-frequencyamplituderesponse.Oneparametermightbeexcellentbuttheremaybesignificantdistortioncreatedelsewhere.Multiphysicsmodelingencompassesalldimensionsthataffectourperceptionofsound,suchasthetime-domainresponseandnonlineardistortion.”
ThesimulationsmadeitpossiblefortheengineersatWATtotweakdesignparametersinordertooptimizeoverallperformance.Ultimately,theywereabletopredictwhatwascausingspikesinthefrequencyresponseandsmoothoutthesignalforbetterfidelity.
“Thisrepresentedahugecostandtimebenefitforus,”saysRoberts.“Wewentfrommakingmultipleprototypesbyhandeachweektosimplydialingupanewoneinthesoftware.Inadditiontosettlingonafinaldesignwe’reveryhappywith,itisnoweasyforustocustomizeourtransducersforclients’individualrequirements.”
Marmo’steamcomparedeachmodelwithphysicalmeasurementsprovidedbytheWATdesignteam.“Thesimulationresultswereastoundinglyclosetothephysicalmeasurements,”commentsDanAnagnos,CTOatWAT.“Thatwasprobablythemostexcitingaspect,seeingthesimulationcometolifeandknowingitwasgivingusanaccuratepictureofhowthespeakerwouldperform.”
FreedomandFlexibilitywithaSimulationAppWithsimulationresultsverifiedandWATsatisfiedwiththeirdesign,thenextstepwasforXi
EngineeringtoputWATincontroloffurthermodeling.TheApplicationBuilderavailableinCOMSOLsoftwareenabledMarmo’steamtobuildanappfromtheirsimulationandhostitonline.
Theapp’sinterfaceallowsuserstochangecertaininputstotestchangestoanumberofparameters,suchastheDCbias,ACsignallevel,frequencyrangeandresolution,materialproperties,speakersize,wiremeshshapeandsize,andspacerplacement(Figure3).Theoriginalmodelsetupisnotaccessiblefromtheapp;instead,itallowsuserstorunfurthertestswithoutneedingtolearnthesoftware.
“ProvidingWATwithasimulationappremovedtheneedforthemtopurchasethesoftwareorappointanexperienceduser,”Marmosays.“Simulationappsputourcustomersincharge,sotheydon’thavetocomebacktousforsmallchangesandtheycantestexactlywhattheywant.Italsofreesustoexplorenewchallenges,ratherthanworkingonvariationsofthesameproblem.”XiEngineeringexpectstousecomputationalappsmoreandmoreinthecourseofitsworkforothercustomers.
Figure3.Bottom:TheappdevelopedbyXiEngineeringallowsengineerstovaryparametersrelatedtofrequency,electricalinput,speakerdimensions,andpropertiesofthemembrane,spacer,andwiremesh.Resultsgivethesoundpressurelevelsfordifferentcases,membranedisplacement,frequencyresponsetodifferentDCbiases,andacomparisonofthesimulateddesignagainstexperimentalresults.
Top:TheappissharedthroughtheCOMSOLServer™productandaccessiblefromawebbrowser.
WATisdoingthesame,sharingtheappwiththeirowncustomers—companieswantingtofindtheHPELtransducerbestsuitedtotheirparticularheadphonedesigns.“TheteamatXiEngineeringhavebeensuperb.Theyhavedeepexpertiseandhelpedtounpackthecomplexityofourproduct,”addsRoberts.“TheintuitiveappthatXidevelopedforusisanadditionalbonus.Withoutrevealinganyintellectualpropertywecangiveourownclientsaccesstoourdesignthroughtheapp,sotheycantestandincorporatethetechnologyintotheirownhigh-endheadphones.”